One type of magnetoresistive head known as a spin-valve type magnetoresistive head includes a magnetoresistive element, which generally consists of a first ferromagnetic layer, a nonmagnetic intermediate layer and a second ferromagnetic layer (this type of magnetoresistive head is disclosed, for example, in U.S. Pat. No. 4,949,039). The electric resistance of the magnetoresistive element changes in accordance with the angle difference between the magnetic directions of the first and the second ferromagnetic layers.
In a typical spin-valve type magnetoresistive head, one of the two ferromagnetic layers serves as a pinned layer and is fixedly magnetized by the magnetic exchange coupling of an anti-ferromagnetic layer formed directly on the pinned layer. The other ferromagnetic layer typically serves as a free layer and is initially magnetized perpendicular to the magnetization direction of the pinned layer. The magnetization direction of the free layer then rotates towards or away from the magnetization direction of the pinned layer under the influence of the magnetic field from a recording medium, thereby changing the electric resistance of the magnetoresistive element in the head.
In commonly assigned U.S. Pat. No. 5,850,323 issued on Dec. 15, 1998, the magnetoresistive element of a head is described as having a Ta bottom underlayer, a NiFe underlayer, a NiMn anti-ferromagnetic layer, a NiFe pinned layer, a Cu nonmagnetic intermediate layer, and a NiFe free layer formed in the given order on a substrate. This arrangement in which a NiFe compound is used as the underlayer for the anti-ferromagnetic layer has two disadvantages with the increase in the recording density of the recording medium.
One problem is the current loss caused by the anti-ferromagnetic layer and the NiFe underlayer. As the magnetic field from the recording media is lowered by high-density recording, the amount of current change in the magnetoresistive element resulting from the magnetoresistive effect is also lowered. The anti-ferromagnetic layer and the NiFe underlayer do not contribute in producing the magnetoresistive effect, and therefore, any current loss through these layers results in an inefficient magnetoresistive effect.
Another problem is the deterioration of bias characteristics of the magnetoresistive head. The magnetic field generated by the sense current of the magnetoresistive head tends to magnetize the NiFe underlayer in the same direction as the pinned layer. As a result, the free layer is affected by the magnetostatic coupling of not only the pinned layer, but also by the NiFe underlayer. This prevents the magnetization directions of the free and the pinned layers from moving in desired angles, which deteriorates the bias characteristics.
Thus, there is a need for a magnetoresistive head having an efficient magnetoresistive effect and good bias characteristics.
Accordingly, one object of this invention is to provide a new and improved magnetoresistive head which prevents current loss in the anti-ferromagnetic layer and the underlayer that do not contribute to the magnetoresistive effect.
Another objective of this invention is to provide a new and improved magnetoresistive head which prevents the underlayer of the anti-ferromagnetic layer from affecting the magnetic direction of the free layer.